The dc electric motor most commonly used to drive an electric vehicle is the series wound motor. There has been, however, considerable recent interest in the shunt-wound motor (or, as it is frequently referred to, the separately excited motor) for these purposes.
For either type of motor, it is, of course, now well known to regulate motor power (i.e., speed and torque) by using solid state thyristor devices in a chopper circuit such that variations in the markspace ratio of the controlled thyristor determine the net effective power applied to the motor. Commonly, the thyristors (silicon controlled rectifiers, or SCRs) are gated into conduction at a periodic rate determined by a gate control circuit responsive to the relative position of an accelerator pedal.
It is characteristic of SCRs that, once they have been gated into conduction, they remain in that state until a zero or reverse polarity has been applied between the anode and cathode terminals, i.e., gate control becomes ineffective. In a dc traction motor control, to turn off, or commutate, the main power controlling thyristor, a commutating capacitor, having a stored reverse voltage in terms of the thyristor, is connected at an appropriate time such that the reverse polarity renders the thyristor non-conducting. Typically, a second control thyristor is gated to discharge the commutating capacitor and carry out the commutation of the main thyristor. When applied to control of a shunt-wound traction motor, however, a unique commutation problem arises, in some instances, upon changing the operating mode of the traction motor.
In that regard, the electric vehicle traction motor may be operated in either a motoring mode, wherein it is desired that the vehicle be propelled along, or in a braking mode, wherein the vehicle is being decelerated. Braking is most generally carried out electrically by reversing direction of the motor in combination with appropriate operation of the accelerator pedal. For a shunt-wound motor, it has been found that a commutation failure may occur whenever the motor is switched to a motoring mode from a braking mode, particularly at such times as the motoring mode preceding the braking mode is characterized by high speed, low torque operation.
A commutation failure in this situation arises as a result of the fact that the charge stored on the commutating capacitor during the motoring mode is insufficient to commutate the relatively high current resulting from the braking operation. When a commutation failure occurs, a line contactor or other device must open the circuit to clear the fault, and then must reclose for normal operation. This causes an undesirable discontinuity in a vehicle's deceleration and produces early deterioration of contactor tips.
Although provision could be made to delay the gating of the controlled thyristor until the braking current decays to a level at which commutation can be assured, this leads to a significant discontinuity in the deceleration profile, producing a drifting feel in the handling characteristics of the vehicle.
Accordingly, it is among the objects of the present invention to provide a commutating circuit and method of operation thereof by which the commutating capacitor charge can be made to track the actual braking current flowing in the motor armature so that transfer from the braking mode to the motoring mode can be accomplished at any level of current without concern for commutation failure.